I suspect the answer is because we don't invest enough in training durhamjen, same with nurses, midwives etc. It's 'cheaper' to import them from poorer countries who have invested in their training.
It's also true that because of the decrease in our manufacturing base, fewer young people were able to access good quality apprenticeships. Remember schemes like the YTS which provided very poor 'training' for young people, rather than the high quality apprenticeships.
Things are improving. One of my grandchildren managed, after A levels and two years of searching, to find an specific engineering apprenticeship. He was lucky, a friend's father introduced him to the small firm. I know several entrepreneurial young folks who have apprentices working with them, hairdressing, beauty therapy, gardening for example. It's a great way for young people to learn a skilled trade and also allow them to grow up with good role models around them.
Our young people will be working into their 70's I expect. Let's allow them longer in the education system than leaving school at 15 because they weren't "bright" enough for the grammar schools.